The present invention relates to a method of binarizing image data representative of a document image read by an image reader in a manner optimal for the characteristic of the image. More particularly, the present invention is concerned with a method of determining the image characteristic of image data to be processed.
Reading a document by a scanner or similar image reader and reproducing the resultant digital image data by a two-level output type printer is a common practice in the imaging art. A prerequisite with such a procedure is that halftone image areas and character image areas included in the digial image data be separated from each other, and each be binarized in a particular manner. To meet this prerequisite, means for distinguishing the halftone image areas and character image areas is indispensable. Japanese Patent Laid-Open Publication (Kokai) No. 146067/1986, for example, discloses a method which applies orthogonal transform to a given image area to determine the frequency components contained therein. This method is derived from the fact that high-frequency components and low-frequency components are predominant in character image areas and halftone image areas, respectively. Specifically, the kind of an image area is determined in terms of the power of high-frequency components. However, the decision by such a method which relies on the power of high-frequency components is not accurate. For example, when a portion containing high-frequency components exists in a halftone image area, the above-mentioned method would erroneously regard the halftone image area as a character image area.